For thermodynamic reasons, it is necessary to increase the fresh steam temperatures in order to increase the efficiency of modern steam turbine plants. At present, steam turbines are designed and produced for fresh steam temperatures of approximately 630° C. and fresh steam pressures of approximately 300 bar. The selection of the materials for the rotor and for the housing plays a significant role. It would appear to be possible to use nickel-based alloys as high-temperature materials for planned fresh steam temperatures of 700° C. The rotor and the housing of a steam turbine suitable for 700° C. could therefore be produced from a nickel-based alloy, though this would be a very expensive solution.
In high-pressure turbine sections, the materials in the vicinity of the inflow region are subjected to extreme thermal loading. In the exhaust steam region of the high-pressure turbine section, the temperature and the pressure of the fresh steam is low in relation to the temperature and the pressure of the fresh steam. It is therefore not imperatively necessary to use the expensive nickel-based alloy in the exhaust steam region.
It is therefore conventional to produce high-pressure turbine sections from different materials. It would thus be possible, for example, for the rotor to be formed as a welded structure, with a nickel-based alloy being used in the fresh steam region and a conventional material being used in the exhaust steam region. This would lead to lower overall production costs. A high-pressure turbine section produced in this way would withstand the loadings which occur in operation. However, the steam temperatures in the exhaust steam region of the high-pressure turbine section are comparatively high during a period of idle operation or low-load operation, as a result of which the conventional material is subjected to too great a thermal loading. This problem occurs in particular during a hot start, since the fresh steam temperatures cannot be arbitrarily reduced in order to limit the thermal loading of the incoming flow.
DD 148 367 describes a method for lowering the work capacity of the steam during a load-shedding process, wherein the solution consists in admixing water to the fresh steam via injection nozzles, thereby reducing the temperature of the steam.